The present application relates to a battery pack inhibited from being charged or discharged depending on the degree of deterioration of each secondary battery included therein and a method of controlling the battery pack.
Recently, portable electronic apparatus such as notebook type PCs (personal computers), portable telephones, and digital cameras have spread, and lithium ion secondary batteries are widely used as power sources for such apparatus because of their advantages such as high outputs, high energy densities, compactness, and small weights.
Lithium ion secondary batteries have high energy densities when compared to other types of secondary batteries utilizing, for example, nickel/cadmium or nickel/hydrogen. It is therefore very important to keep such batteries at a sufficient level of safety. For this reason, a battery pack utilizing lithium ion secondary batteries includes a protection circuit for inhibiting charging or discharging to prevent over-charging, over-discharging, or an over-current.
However, it is inappropriate to say that a battery pack can be kept at a sufficient level of safety by such a protection circuit alone, for example, when lithium ion secondary batteries therein have deteriorated as a result of the operation of the battery pack.
It is generally known that deterioration of a lithium ion secondary battery proceeds as a result of repeated charging and discharging and that the deterioration consequently increases the internal resistance of the secondary battery. When the internal resistance of a secondary battery increases as a result of deterioration, the battery undergoes a significant voltage drop corresponding to the increase in the internal resistance, and the capacity of the battery is therefore reduced even if charging conditions are kept unchanged.
Let us now assume that a plurality of secondary batteries (battery cells) are connected in series in a battery pack. When the battery pack having a plurality of battery cells connected in series is repeatedly charged and discharged, each of the battery cells undergoes deterioration, and the degree of deterioration may vary between the battery cells. When the battery cells have different battery capacities because of different degrees of deterioration between the battery cells, what is called battery balance is lost. Then, a battery cell which has deteriorated to a significantly high level compared to the other battery cells may be at an undesirably low level of safety. In such a case, it is desirable to take measures such as interrupting charging or discharging of the battery pack in consideration to safety of a user.
For example, Patent Document 1 (JP-A-2007-335337) discloses a technique for detecting the state of cell balance between a plurality of battery packs used in a power supply apparatus and inhibiting charging or discharging when it is determined that cell balance has been lost